Oxides of nitrogen have been identified as contributors to smog. A NOx adsorber catalyst placed in the exhaust system of an internal combustion engine can adsorb substantial amounts of oxides of nitrogen from engine exhaust passing through it so that they do not escape to the ambient atmosphere. A NOx adsorber catalyst may offer an advantage over those catalysts that are intended to be used with internal combustion engines running mainly at stoichiometric because it can allow an engine to run on a relatively leaner fuel-air mixture that renders the engine somewhat more efficient than when running at stoichiometric or richer.
As the engine operates, the NOx adsorber catalyst adsorbs more and more oxides of nitrogen and eventually must be regenerated. Generally speaking, a NOx adsorber catalyst adsorbs NOx in engine exhaust when the engine is running relatively leaner. The catalyst is regenerated by temporarily enriching the fuel-air mixture, resulting in release of the adsorbed oxides of nitrogen as gaseous nitrogen, carbon dioxide and water through catalytic aided chemical processes.
Some known methods for enriching a fuel-air mixture for regeneration of a NOx adsorber catalyst include the use of intake throttling, variable valve actuation, and external exhaust gas recirculation (EGR) in conjunction with control of engine fueling, such as by post-injection. The process may also be controlled by the use of feedback from a suitable exhaust sensor. A typical practice has been to control the rich-lean modulation of engine fueling used for NOx adsorber catalyst regeneration by means of timing functions, for example 30 seconds of lean and 3 seconds of rich.